Self‐Assembly of Lamellae‐in‐Lamellae by Double‐Tail Cationic Surfactants

Abstract The molecular structures of surfactants play a pivotal role in influencing their self‐assembly behaviors. In this work, using simulations and experiments, an unconventional hierarchically layered structure in the didodecyldimethylammonium bromide (DDAB)/water binary system: lamellae‐in‐lamellae is revealed, a new self‐assembly structure in surfactant system. This self‐assembly structure refers to a lamellar structure with a shorter periodic length (inner lamellae) embedded in a lamellar phase with a longer periodic length (outer lamellae). The normal vectors of these two lamellar regions orient perpendicularly. In addition, it is observed that this lamellar‐in‐lamellar phase disappears when the two tails of the cationic surfactants become longer. The formation of the lamellar‐in‐lamellar architecture arises from multiple interacting factors. The key element is that the short tails of the DDAB surfactants enhance hydrophilicity and rigidity, which facilitates the formation of the inner lamellae. Moreover, the lateral monolayer of the inner lamellae provides shielding from the water and prompts the formation of the outer lamellae. These findings indicate that molecular structures and flexibility can profoundly redirect the hierarchical self‐assembly behaviors in amphiphilic systems. More broadly, this work presents a new strategy to deliberately program hierarchical nanomaterials by designing specific surfactant molecules to act as tunable scaffolds, reactors, and carriers.


Supporting Figures and Tables
profiles between headgroups of DDAB.These indicate that at higher temperatures, the stability of the lamellar-in-lamellar structure is compromised.Specifically, we observed that as the temperature increases, the outer lamellae (L=) tend to come into closer contact and undergo fusion.This phenomenon can be attributed to the increased thermal energy in the system with higher temperature, leading to enhanced molecular mobility and the merging of adjacent lamellae.Additionally, the well-defined long-range ordered arrangement is weakening and the molecular conformation of DDAB becomes more disordered under higher temperatures.Analogous to the impact of temperature, our results revealed that higher salt concentrations detrimentally affect the stability of the lamellar-in-lamellar structure, as evidenced by both qualitative observations in shapnots, density maps and quantitative analyses through RDF profiles.

Figure S1 .
Figure S1.Critical micelle concentration of double-tail surfactant molecule with different tail length.SW represents the standard water model, 1 and the PW refers to the polarizable water model. 2PME is the Particle Mesh Ewald algorithm and RF is the Reaction Field algorithm.Both algorithms are using for computing the long-range electrostatic interaction.

Figure S2 .
Figure S2.(a-c) Snapshots and (d-f) POM images of (a, d) 1 wt%, (b, e) 5 wt%,and (c, f) 20 wt% DDAB/water binary systems; (g) SAXS profiles and (h) RDF profiles between headgroups of the corresponding dilute DDAB/water binary systems.As the figures shows, the lamellar-in-lamellar structure is not observed in these lower concentrations.This outcome suggests that the formation and stability of the lamellar-in-lamellar structure are highly dependent on the DDAB concentration.

Figure S3 .
Figure S3.(a) Density and (b) potential energy of different systems as a function of the simulation time.The r1, r2 and r3 represent the replica-1, replica-2 and replica-3, respectively.

Figure S4 .
Figure S4.Snapshots of 70 wt% DDAB/water binary system from (a) perspective view, (b) front view, (c) side view, and (d) top view; (e) snapshot of singular layer of 70 wt% DDAB system, where the red beads denote the headgroups (Q 0 ) of the surfactant's outer monolayer, while the orange beads represent the tails (C 1 and C 2 ) of the surfactant's outer monolayer.

Figure S12 .
Figure S12.(a, b) Snapshots and(c, d) 2D density maps of headgroup of 60 wt% DDAB/water binary system using box sizes of (a, c) 24 nm and (b, d) 36 nm (Z dimension); (e) RDF profiles between headgroups of DDAB.As depicted in the provided image, even with these different box sizes, DDAB forms lamellar-in-lamellar structures in both cases with box sizes of 24 nm and 36 nm, indicating robust hierarchical organization.The RDF analysis further confirms the presence of this hierarchical structure with a well-defined longrange ordered arrangement.

Figure S13 .
Figure S13.(a, b) Snapshots and(c, d) 2D density maps of headgroup of 60 wt% DDAB/water binary system at temperatures of (a, c) 308 K and (b, d) 318 K; (e) RDF profiles between headgroups of DDAB.These indicate that at higher temperatures, the stability of the lamellar-in-lamellar structure is compromised.Specifically, we observed that as the temperature increases, the outer lamellae (L=) tend to come into closer contact and undergo fusion.This phenomenon can be attributed to the increased thermal energy in the system with higher temperature, leading to enhanced molecular mobility and the merging of adjacent lamellae.Additionally, the well-defined long-range ordered arrangement is weakening and the molecular conformation of DDAB becomes more disordered under higher temperatures.

Figure S14 .
Figure S14.(a-c) Snapshots and(d-f) 2D density maps of headgroup of 60 wt% DDAB/water binary system at different NaCl concentrations of (a, d) 0.01 M, (b, e) 0.05 M, and (c, f) 0.1 M; RDF profiles of (g) headgroup to headgroup and (h) headgroup to water.Analogous to the impact of temperature, our results revealed that higher salt concentrations detrimentally affect the stability of the lamellar-in-lamellar structure, as evidenced by both qualitative observations in shapnots, density maps and quantitative analyses through RDF profiles.

Table S2 :
Summary of DOAB and DPAB/water binary system.Surfactant wt% N surfactant N water Box Size (nm 3 ) Temperature (K)